SELECTION OF PEAT LANDS FOR DIFFERENT USES 7 
plant associations which varied according to climate and occupied 
ponds or formed marshes and swamp forests (pi. 2; also fig. 2). 
Plate 2, A, illustrates aquatic vegetation forming a layer of fine-tex- 
tured peat by sedimentation in open water. The marginal sedge 
marsh gives rise to a layer of fibrous radicellate peat material. 
Plate 2, B, shows a sphagnum bog from which a layer of coarsely 
fibrous moss peat is derived. The dwarfed growth of conifers is a 
reaction to the adverse conditions for trees rooting in a la}^er of 
sphagnum peat; it is difficult for tree seedlings to become estab- 
lished in coarsely fibrous types of material. The remnant of a for- 
mer larger lake is shown, which is nearly filled with sedimentary 
peat. Plate 2, C, represents a swamp forest of tamarack and red 
maple, forming a layer of woody peat. The young trees are seen 
growing on sedge peat well above the water level of an adjacent 
pond. Sedge marshes, when they have reached a certain stage of 
drainage, are often very rapidly covered by forest through natural 
reproduction. 
The several social groups of vegetation were related to the edaphic 
environment chiefly with regard to the general moisture conditions. 
Mineral soils, topography, and successional relations of plant com- 
munities do not appear to have been of any considerable significance 
in determining the character, continuity, and thickness of a layer of 
peat. Neither do these factors account for the number of layers 
superimposed upon one another or for their arrangement in the dif- 
ferent profile classes of peat areas. The variations in the profiles of 
peat areas seem to show a much closer relationship to climatic condi- 
tions and consequent changes of level in ground water (14-)- The 
formation of peat deposits in ancient times probably has not been 
very different. The thickness or depth of peat areas within the 
older morainal belts, as well as those of the coal periods in general, is 
greater, it seems, by reason of the greater length of time for undis- 
turbed accumulation of peat materials. 
The individual characteristics and properties of the several kinds of 
peat, in the original and in the modified condition as peat soils, are 
traceable to their respective botanical composition. The dimensions 
of the plant remains may be regarded as the most important point 
for physical comparison. They concern the relation between coher- 
ence or consistency and moisture content. As regards organic ma- 
terial the proportions of the nitrogenous and carbonaceous, the 
lignose, cellulose, pentose, and other substances, the fibrous and non- 
fibrous fractions of a peat layer, account quantitatively for chemical 
differences. The physiological properties of this class of organic sub- 
stances rest on a special relationship to plants and microorganisms, 
which depends in the most intimate way on the nutritive value of 
the compounds, it has not been fully determined to what extent 
and in what condition the respective layers of peat may serve as a 
source of plant-food constituents or how humus is formed from them. 
There is already considerable information on the physical and chem- 
ical characteristics of the several kinds of peat. Some of the out- 
standing data have been tabulated in papers which deal with the 
quality and value of important types of peat (8, 12). Photographs 
that aid in the identification of different layers of peat are shown in 
Plates 3 to 6. Knowledge of these materials would be greatly 
advanced, however, if analytical and experimental work upon them 
